A device as shown in FIG. 10 has already been well-known as a conventional fixing device for fixing toner images having undergone electrostatic transfer onto a transfer paper sheet in a copying machine, a printer or the like. The fixing device shown in FIG. 10 is a so-called two-roller type fixing device, which is configured so that a paper sheet 1 with toner 5 having undergone electrostatic transfer onto the sheet is pressed by two rollers, i.e., a fixing roller 2 that is on a side of the paper sheet 1 having toner images electrostatically transferred (that will be referred to as toner fixation surface side, hereinbelow) and that includes a heat source 4 and a pressure roller 3 that is placed on a side opposite to the toner fixation surface side with respect to the paper sheet 1 (that will be referred to as opposite surface side, hereinbelow) and in parallel with the fixing roller 2. Then, the toner 5 is fixed on the paper sheet 1 by heat and pressure.
In the fixing roller 2, a nip part is required to be formed widely in order to ensure fixity. For that purpose, the fixing roller 2 is composed of a cylindrical metal core 6 formed of metal and having a surface covered cylindrically with thick rubber material 7. The heat source 4 is placed along a center axis of the metal core 6. Similarly, the pressure roller 3 is composed of a cylindrical metal core 8 formed of metal and having a surface covered cylindrically with thick rubber material 9, in order to ensure pressurizing and nipping abilities.
In the above described instance that the fixing roller 2 being thick and having a large heat capacity is used, it takes about six to seven minutes as a period of time (which will be referred to as “warm-up time (WUT)” hereinbelow) since a power to the heat source 4 is turned on until a surface of the rubber material 7 has a specified temperature that makes copying possible, and there is caused a defect in that long waiting time is required when the power is turned on.
In order to eliminate the defect of the long warm-up time and reduce the warm-up time, so-called three-roller type fixing devices have been developed. FIG. 11 shows a three-roller type fixing device disclosed in JP 2002-244484 A (Patent Literature 1).
As shown in FIG. 11, the fixing device 101 disclosed in Patent Literature 1 has a fixing roller 102, a heating roller 103, a fixing belt 104 stretched between the fixing roller 102 and the heating roller 103, and a pressure roller 105 that is placed so as to face the fixing roller 102 and that is in press contact with the fixing roller 102 with the fixing belt 104 between.
The heating roller 103 is cylindrically formed of thin metal material and has a halogen heater 106 as a heat source in rotation center part thereof. The fixing roller 102 is placed on the toner fixation surface side in parallel with the heating roller 103 and has a metal core 102a that is cylindrically formed of metal and a silicone rubber layer 102b that is formed on an outer circumferential surface of the metal core 102a. The pressure roller 105 is placed on the opposite surface side so as to face the fixing roller 102 and has a metal core 105a that is cylindrically formed of metal and a silicone rubber layer 105b that is formed on an outer circumferential surface of the metal core 105a, like the fixing roller 102. The pressure roller 105 has a halogen heater 107 as a heat source in rotation center part thereof. The fixing belt 104 is driven by the heating roller 103 and the fixing roller 102 and, also when rotated, is heated by the heating roller 103 up to generally the same level as the heating roller 103 is.
In the fixing device 101 disclosed in Patent Literature 1, the heating roller 103 including the halogen heater 106 has a small heat capacity because the device is cylindrically formed of thin metal material, and the fixing belt 104 placed on the heating roller 103 also has a small heat capacity because the belt has a small volume. Therefore, a temperature of the fixing belt 104 can rapidly be increased and, even in a condition of the belt being rotated, the temperature of the fixing belt 104 is rapidly increased once it is brought into contact with the heating roller 103. Thus, the fixing belt 104 having an increased temperature is rotated and reaches the nip part on the pressure roller 105, so that fixation is allowed.
That is, the three-roller type fixing device has the warm-up time on the order of 30 to 45 seconds and has a benefit in that the waiting time on power-on is reduced.
The conventional fixing devices described above, however, have problems as follows.
Namely, a fact in common with the two-roller type fixing device and the three-roller type fixing device is that the pressure roller 3 and the pressure roller 105 used in both the fixing devices are composed of the metal core 8 and the metal core 105a cylindrically formed of metal and having surfaces covered with the rubber material 9 and the silicone rubber layer 105b, respectively, that have medium or large thicknesses and therefore have large heat capacities, for ensuring certain nip widths. Accordingly, increases in temperatures of the pressure roller 3 and the pressure roller 105 upon power-on are mild, as in the fixing roller 2 and the fixing roller 102. A rise-up of surface temperature of the pressure roller 3 is further retarded in the fixing device that lacks a heater as a heat source inside thereof, as in the pressure roller 3 in the fixing device shown in FIG. 10.
And now, in a copying machine having the fixing device installed therein, fixation performance immediately after power-on (e.g., at start of work in the morning or the like) is chiefly dominated by the surface temperature of the fixing roller (surface temperature of the fixing belt in the three-roller type fixing device), and thus, the warm-up time at start of work in the morning and the like is determined by time that elapses until the surface temperature of the fixing roller (the surface temperature of the fixing belt) reaches a specified temperature (typically on the order of 180 to 200° C.).
That is because the temperature of the fixing roller that resides on the side where toner is fixed on paper sheets has greater influence on improvement in fixation performance than the temperature of the pressure roller that is brought into press contact with the paper sheets from back side of the paper sheets opposite to the toner fixation side. In order to achieve satisfactory fixation performance at start of work in the morning, accordingly, the temperature of the fixing roller is required to be dominantly increased.
In the fixation device, typically, 80% to 100% (800 to 1000 W) of total wattage (on the order of 1000 W) is used for heating of the heater in the fixing roller and remaining 0% to 20% (0 to 200 W) thereof is used for heating of the heater in the pressure roller. In the fixing device at start of work in the morning, as shown in FIG. 12, therefore, the temperature of the fixing roller rapidly increases, while the temperature of the pressure roller calmly increases. In this regard, FIG. 12 is an example of the three-roller type fixing device in which all of total wattage for fixation is used for heating of the heater in the fixing roller and in which no heater is integrated in the pressure roller.
When the power to the fixing device is turned on at a room temperature on the order of 20° C., in the example of FIG. 12, the temperature of the fixing roller is raised to 200° C. in about 30 seconds. On the other hand, the temperature of the pressure roller is only raised to about 50° C. in about 30 seconds because no heater is integrated in the pressure roller. The heater in the fixing roller is turned off the moment the temperature of the fixing roller reaches the controlled temperature of 200° C., and temperature control operations are thereafter repeated in which the heater is turned on at temperatures lower than 200° C. (controlled temperature) and is turned off at temperatures not lower than 200° C. Though the pressure roller is increased in temperature by being heated by the fixing roller during the temperature control operations, it takes about 120 seconds for the pressure roller to be increased to the temperature on the order of 120° C., which is a saturation temperature of the pressure roller.
There is also caused a problem in that curl condition of paper sheets is deteriorated by a temperature difference between the temperatures of the fixing roller and the pressure roller at start-up of the fixing device as shown in FIG. 12 (that is, the difference of 150° C. (200° C.-50° C.) between the temperature of the fixing roller of 200° C. and the temperature of the pressure roller of 50° C.).
FIG. 13 show a mechanism of curl of a paper sheet. When a paper sheet 111 having unfixed toner (not shown) deposited thereon is conveyed to the fixing device, the unfixed toner is fixed on the paper sheet by heat and pressure in a nip part 114 between a fixing roller 112 and a pressure roller 113. At a moment as the above start-up, on condition that a surface temperature of the fixing roller 112 is higher than a surface temperature of the pressure roller 113 on this occasion, as shown in a upper part of FIG. 13, majority of moisture 115 residing in the paper sheet 111 is vaporized from a side of the fixing roller 112 in the nip part 114 and the moisture is not so much vaporized from a side of the pressure roller 113 having the lower temperature. As a result, the moisture on the side of the fixing roller 112 in the paper sheet 111 decreases, while the moisture on the side of the pressure roller 113 does not decrease so much, so that a nonequilibrium state is established. Subsequently, the moisture on the side of the pressure roller 113, as well as moisture residing in center part with respect to a longitudinal section of the paper sheet 111, moves to the side of the fixing roller 112 all at once.
As shown in a lower part of FIG. 13, consequently, the paper sheet 111 reaches a state in which the side of the fixing roller 112 is rich in moisture and in which the side of the pressure roller 113 is poor. Thus, the concentrated moisture expands the side of the fixing roller 112 in the paper sheet 111, while the escaping of moisture shrinks the side of the pressure roller 113. The expansion on one side and the shrinkage on the other side account for the mechanism of the curl of the paper sheet 111. In FIG. 13, the paper sheet 111 exhibits a back curl such that the sheet is bent toward the pressure roller.
The curl of the paper sheet occurs through such a mechanism. Therefore, the greater the temperature difference between the temperatures of the fixing roller 112 and the pressure roller 113 is, the worse the curl of the paper sheet 111 becomes. Accordingly, degree of the curl is great and disadvantageous in a high-temperature high-humidity environment (HH environment) in which there is much moisture in the paper sheet 111, while the degree is little and advantageous in a low-temperature low-humidity environment (LL environment). On the other hand, the fixation performance, which is a capability to melt and fix toner by heat and pressure, is advantageous in the high-temperature high-humidity environment (HH environment), and is disadvantageous in the low-temperature low-humidity environment (LL environment). Therefore, advantages/disadvantages in the fixation performance and the curl condition are in trade-off relation with respect to the environmental conditions.
The temperature difference between the surface temperatures of the fixing roller and the pressure roller can be controlled by a fixing press contact force that is a nip load between the fixing roller the pressure roller. In recent years, there have been appeared fixing devices that have two modes, i.e., a heavy press contact mode with a strong press contact force and a light press contact mode with a weak press contact force, as modes of the fixing press contact force. Among such devices having heavy press contact mode and light press contact mode is an electrostatic recording device disclosed in JP 61-294475 A (Patent Literature 2).
In the electrostatic recording device disclosed in Patent Literature 2, as shown in FIG. 14, a fixing device 121 has a fixing roller 123 including a heater 122 therein and a pressure roller 124 in press contact with the fixing roller 123. The fixing roller 123 is journaled by a main body (not shown) of the device and opposite ends of the pressure roller 124 are supported by levers 125. One end of each lever 125 is rockably supported by a pin fixed on the device main body and the other end thereof is biased by one end of a compression spring 126 in a direction in which the fixing roller 123 is pressed. The other end of the compression spring 126 is supported by a spring receiver 129 supported by one end of a lever 128 that rocks about a fulcrum 127 fixed on bottom part of the device main body.
When an envelope mode is selected as a mode of transfer material and a leading edge of an envelope is detected by a sensor S, a cam 133 is rotated through a stepping motor 130, a worm 131 and a worm wheel 132 so as to raise a position of an engaging point between the cam 133 and the other end of the lever 128. In this manner, the fixing press contact force between the pressure roller 124 and the fixing roller 123 is decreased from that for regular paper.
In an instance of paper such as an envelope being prone to have abnormality such as paper wrinkles or the like, occurrence of such paper wrinkles can be prevented by setting to the light press contact mode as described above. Then, switching between the heavy press contact mode and the light press contact mode changes heat transference between the fixing roller and the pressure roller, so that the temperature difference between the surface temperatures of the fixing roller and the pressure roller can be controlled.
FIGS. 15A and 15B show a mechanism of the temperature difference control according to the modes of the fixing press contact force. In the heavy press contact mode shown in FIG. 15A, the fixing roller 123 and the pressure roller 124 are brought into strong press contact with each other. In the light press contact mode shown in FIG. 15B, by contrast, the fixing roller 123 and the pressure roller 124 are brought into weak press contact with each other. In the fixing roller 123 and the pressure roller 124, elastic body of rubber is used as surface material in order to ensure formation of the nip part. Thus, the nip width is widened in the heavy press contact mode and is narrowed in the light press contact mode. Accordingly, heat transfer between the fixing roller 123 and the pressure roller 124 is activated in the heavy press contact mode with the large nip width (contact area) and is inactivated in the light press contact mode. In the heavy press contact mode, consequently, heat in the fixing roller 123 having a high temperature is more actively transferred to the pressure roller 124 having a low temperature.
As one example, FIG. 16 shows manners of increase in the temperatures of the fixing roller and the pressure roller at start of work in the morning in the three-roller type fixing device. When the power to the fixing device and a heater for the fixing roller are turned on at a room temperature on the order of 20° C., in the example of FIG. 16, the temperature of the fixing roller having a large heater wattage rapidly increases and the temperature of the pressure roller having a small (or no) heater wattage calmly increases in both the heavy press contact mode and the light press contact mode. In the heavy press contact mode with active heat transfer, rising in the temperature of the pressure roller is accelerated because heat from the fixing roller whose temperature is rapidly increasing is actively transferred to the pressure roller. As a result, rising in the temperature of the fixing roller from which heat escapes to the pressure roller is decelerated. In the light press contact mode with inactive heat transfer, by contrast, the rising in the temperature of the pressure roller is decelerated in comparison with the heavy press contact mode. As a result, the rising in the temperature of the fixing roller whose heat is hard to escape to the pressure roller is more accelerated than in the heavy press contact mode.
At a point in time when 30 seconds have elapsed since the heater for the fixing roller was turned on, in the example shown in FIG. 16, the temperatures of the fixing roller/the pressure roller are 170° C./80° C. in the heavy press contact mode and 200° C./50° C. in the light press contact mode.
The conventional electrostatic recording device disclosed in Patent Literature 2, however, has problems as follows.
Provided that the light press contact mode is set as the mode of the fixing press contact force, and that the surface temperature of the fixing roller which dominates the fixation performance is attempted to be higher in order to improve the fixation performance at start-up of the electrostatic recording device, the surface temperature of the pressure roller becomes lower as is understood from FIG. 16. This increases the difference between the surface temperatures of the fixing roller and the pressure roller (the example shown in FIG. 16 results in the difference of 150° C. (=200° C.-50° C.)), and causes the problem in that curl condition of paper sheets is deteriorated as described above.
On the contrary, provided that the heavy press contact mode in which the surface temperature difference between the fixing roller and the pressure roller is small is set in order to improve the curl condition of paper sheets, the rising in the temperature of the fixing roller is decelerated, although the temperature difference between the fixing roller and the pressure roller becomes small. This causes a problem of deterioration in fixation performance at start in the morning or extension of warm-up time.